When users in a variety of industrial applications considered a compressed gas system there were many choices. These systems could serve as plant air systems to operate a wide variety of machine components and control devices. Depending on the pressure and volume requirements of a particular location different compression packages could be used for the application. Each system had its unique advantages and disadvantages. Generally speaking as power costs increased worldwide, a greater focus was placed on multi-stage centrifugal compression systems over positive displacement designs such as screw compressors. The reason for this was that the positive displacement machines became less efficient as they wore, in normal use. In general, the initial efficiency of centrifugal compressor packages was higher than the positive displacement counterparts and the centrifugal compressor efficiency would maintain a nearly constant level over long periods of operation. Centrifugal compressors also offered excellent part load efficiencies and eliminated sliding or rubbing parts, such as in screw compressors, which would cause efficiency loss over time.
Other advantages of centrifugal compressors are high reliability, the availability of oil-free air and ease of maintenance. Some features that made these advantages possible were: non-contact air and oil seals; stainless steel compression elements; high quality gear design using unlimited life pinion bearings; the elimination of the need for oil removal filters; elimination of need to remove wearing parts; and an accessible horizontally split gearbox for quick inspections.
In the past, multi-stage centrifugal compressor units had been sold with inter-stage water-cooling to improve efficiency of the overall system. Use of water-cooled designs involved a host of significant associated costs, especially cooling towers. It also precluded applications of water-cooled centrifugal compressor packages in locations where water was not readily available or prohibitively expensive. Some potential installations also had space constraints that made use of water-cooled centrifugal compressors impossible. Water cooled systems involving cooling towers not only had space and installation cost elements but also required substantial operating costs for things such as make up water, pumping costs, chemicals including glycol to deal with potential freezing problems. Even connection to existing closed loop chilled water systems, assuming they had the requisite capacity, involved significant piping installation expenses and some of the same incremental operating costs previously described.
Multi-stage centrifugal compressor packages have, in the past, been highly engineered to be space efficient. They have been sold as a compact package with the intercoolers below a gearbox that connects all three stages to a single drive motor. The lubrication system reservoir would be provided as a separate casting from the intercoolers and mounted alongside. FIG. 5 illustrates this layout. There the drive motor 10 is connected through a gearbox 12 to the first stage 14, the second stage 16 and the third stage 18 centrifugal compressors. Compressed gas from the first stage 14 enters cooler 20 and passes into the second stage 16 through inlet pipe 22. The second stage 16 has an outlet line 24 into cooler 26 and the third stage, which receives the cooled gas from cooler 26, has its exhaust directed to an after-cooler 28. The final discharge is through line 30 which is directed upwardly adjacent the inlet line 32 to the first stage 14. A control panel 34, which sometimes requires cooling, is at one end of the skid package as is a reservoir for the lubricating oil 36, which has its own cooler (not shown). For noise control and appearance purposes, the skid further comprised a metal paneled enclosure. It should be noted that while a water-cooled system is illustrated in FIG. 5, that Figure is not considered or labeled prior art because one aspect of the present invention is to retrofit such units to air cooled operation with a minimum of modifications. This mode of the invention will be described in more detail below.
Accordingly, with the layout of skids for multi-stage centrifugal compressor packages having gained acceptance in the industry not only for its efficient performance but also for the compactness of the package, a challenge was presented to the named inventors to create an innovative package that would be more economical to install and operate than the previous water cooled designs but would also fit a housing and have a compact size, such as a comparable footprint, for a given driver horsepower. The present invention provides air-cooling as an option on a multi-stage centrifugal compressor package with no significant performance penalty. The present invention is packaged as a unit in a comparably sized enclosure having a footprint not larger than a water-cooled unit having the same driver horsepower. It does not require the space or expense of a cooling tower. The present invention captures the exhaust heat from air-cooling in a variety of ways. The present invention permits optimization of performance and power consumption in an air-cooled environment by matching the cooling capacity to the produced output. Specialized packages can be created for particular applications such as the air separation industry where there is a need for compressed air as well as compressed nitrogen from a single package. The unit can be used to filter the room air in the environment in which it is installed. It can be a retrofit of existing water-cooled units, such as shown in FIG. 5, into an air-cooled system with minimal piping modifications and elimination of the previously necessary cooling tower, if it was exclusively dedicated to cooling duty for the centrifugal compressor package.
In the past, exhaust gas from a second stage water-cooled unit has been used to regenerate air dryers filled with desiccant. This technique is illustrated in U.S. Pat. No. 6,221,130. There were positive displacement compressor packages offered with an air-cooling feature. However, in the realm of centrifugal multi-stage compressor packages, there have never been air-cooled commercial units available. The industry, as well as the end user customers, were convinced that an air-cooled centrifugal multi-stage package could not deliver the efficiency of the known water-cooled designs. The inventors, facing this prejudice, were forced to present technical data from testing such an air-cooled unit to potential customers. Data that is not normally part of ordinary commercial transactions in water cooled designs, such as FIG. 7, had to be given to potential customers to persuade them that the promised results were indeed achievable. Competitors, who offered positive displacement air-cooled units, had failed to seize upon a vast market that had gone unserved for so many years. After rollout of the air-cooled package, the customer response has been unprecedented and there is now interest from competitors to develop competing products.
Part of the difficulty in accomplishing the objective of an air cooled multi-stage centrifugal compressor unit of comparable performance to a water cooled design was to be able to package the entire system in a comparable volume while getting comparable performance. Tube/fin air-to-air exchangers were tested. While such units were operative, they didn't match the cooling performance of the counterpart water-cooled systems then commercially available. They also occupied significantly more space than the water cooled counterparts. The inventors were encouraged by these results and proceeded to further optimize the performance and compactness of the assembly. What resulted was the matching up of the plate fin air cooler type to the multi-stage compressor package in a confined volume. This combination rendered comparable performance to a water cooled unit of identical size while keeping the package size comparable. This became the optimal design for commercial use. These and other features of the present invention will be more readily understood from a review of the preferred embodiment, which appears below.